Experiments have been conducted to determine (1) the diffusive transport rate of labelled rabbit serum albumin in rabbit thoracic aorta in vitro and (2) the effect of pressure driven bulk flow on the transport process. Diffusion coefficients for the labelled albumin were about 50 times lower in the aortic media than in aqueous solutions. Imposition of a 100 mm Hg pressure difference across the aortic wall resulted in a 5-10 fold increase in the albumin flux compared with the purely diffusive condition. A mathematical transport model has been developed to facilitate the interpretation of the experimental data. A comparison of theoretical and experimental results suggests that the albumin interacts with tissue components as it is being transported through the aortic media. Further studies are planned to determine the permeability of rabbit aortic wall to beta-lipoprotein.